1. Field of the Invention
The present invention relates to an automotive automatic transmission and, more particularly, to a device for controlling the operation of a lock-up clutch incorporated in a multi-speed automatic transmission which is composed of a main transmission unit and a sub transmission unit.
Generally, an automatic transmission has a torque converter and a planetary speed changing gear mechanism. The planetary speed changing gear mechanism is composed mainly of an overdrive (O/D) planetary gear unit, a front planetary gear unit and a rear planetary gear unit. The planetary speed changing gear mechanism performs, under the control of two solenoid valves and three shift valves, a speed change over four forward speeds and one reverse speed.
2. Description of the Related Art
In the known automatic transmission of the type described, the torque converter incorporates a lock-up clutch which is controlled such that it can lock-up the torque converter when the second or higher speed gear is selected. To this end, the controller for the lock-up clutch delivers, as the controlling pressure for controlling the lock-up relay valve and the solenoid valve thereof, a hydraulic pressure which is taken off of a line leading to a second brake B.sub.2.
The inventors have already proposed, as in Japanese Patent Laid-Open Publication No. 37140/1982, an automatic transmission in which, by using the overdrive planetary gear unit as a sub transmission unit and the front and rear planetary gear unit as a main transmission unit, six forward speeds are attained through a suitable control of three solenoid valves and three shift valves.
More specifically, in this multi-speed automatic transmission having the sub-transmission unit and the main transmission unit, the sub-transmission unit provides two speeds, i.e., the overdrive speed and the direct-coupling speed, while the main transmission unit selects one of 1st, 2nd and 3rd speed gears, so that six speeds are obtained by combining both transmission units. However, if the lock-up relay valve and the solenoid valve are controlled by the hydraulic pressure derived from the second brake B.sub.2 as in a known automatic transmission, a problem will be encountered in that, when the sub-transmission unit is upshifted from the direct-coupling speed to the overdrive speed while the main transmission unit selects the 1st speed gear, i.e., when the automatic transmission as a whole selects the second speed amongst the six speeds, the control pressure cannot be supplied to the lock-up relay valve and the solenoid valve, so that the lock-up clutch cannot be engaged, resulting in a disadvantage in that the lock-up clutch cannot be fully utilized.